Azo compound (Azo dye)
Azo compounds, also known as azo dyes, are synthetic organic chemical compounds characterized by the presence of the azo group (–N=N–) in their molecular structure. These dyes account for approximately 60 to 70 percent of all dyes used in textiles and foods, with a variety of forms including monoazo, disazo, and trisazo dyes. Azo dyes are primarily aryl compounds, though alkyl azo compounds are less common due to their instability. They are widely used to color textiles, leather, and some tattoo inks, and they can also serve as pigments in plastics and paints.
However, the use of azo dyes raises health concerns, as some are known carcinogens. The potential for these dyes to degrade into harmful aromatic amines has prompted regulatory scrutiny, particularly in the European Union, where certain azo dyes have been banned from consumer products. Additionally, studies have linked some azo dyes in food to behavioral changes in children, leading to further recommendations for their removal from food products. The production of azo dyes involves a chemical process called diazotization, which allows for the customization of colors through modifications to their chemical structure.
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Azo compound (Azo dye)
Azo dyes are synthetic organic chemical compounds that have nitrogen as the azo group—two adjacent nitrogen atoms between carbon atoms—in the molecular structure: –N = N–. The word azo comes from azote, the French word for nitrogen.
Azo compounds make up 60 to 70 percent of all dyes used in foods and textiles. Most of the dyes contain one azo group. Disazo dyes contain two, and trisazo dyes contain three. Some dyes may contain even more azo groups. Azo compounds may be aryl or alkyl. Aryl azo compounds are the most common; alkyl azo compounds are less stable and not as frequently used commercially.
Azo dyes are commonly used to color textiles and leather goods. Some are known carcinogens (cancer-causing agents), and this has raised concerns about their safety in clothing and toys. A few azo dyes have been banned in consumables. Some tattoo inks, particularly reds, contain azo dyes. Azo compounds are also used to make pigments, another type of coloring agent. Dyes are soluble compounds and are usually applied to textiles in a water-based solution, while pigments are insoluble compounds combined with a medium of appropriate particles—such as dry pigment powders. Pigments are often used to color plastics and paints.
Background
For centuries, dyes were produced using natural products. Most natural dyes are made from plant sources—indigo, for example, is made using plant leaves. Some are made from minerals, while others use insects, such as the cochineal scale insect. Purple dyes required mollusks that were not easily obtained and produced only a small amount of the substance needed, so only royals could afford purple clothing.
Synthetic dyes were discovered by accident during the nineteenth century. William Henry Perkin, who was a student and lab assistant of August Wilhelm von Hofmann, was trying to synthesize quinine. Quinine, which is obtained from the bark of the cinchona tree, is used to treat an infection called malaria. It was expensive, and the available supply did not meet the need for it. In 1856, Perkin was working with an organic compound, aniline, when he discovered it produced a beautiful purple color. Few purple dyes had been produced from natural sources, and this discovery drew attention. Queen Victoria wore a lavender silk gown colored using the resulting dye, called mauveine, to the Royal Exhibition of 1862 and caused a sensation. Chemists began working to develop more synthetic dyes. Researchers at the Royal College of Chemistry in London discovered diazo compounds, which led to the development of the azo compounds. In 1863, German chemist Carl Alexander von Martius—who also worked for Hofmann—developed Bismarck Brown, the first true azo dye.
Modern commercial dyes and pigments are classified by their chemical structures and method of application. They are registered with the Society of Dyers and Colourists, which maintains a list known as the Color Index (CI).
Overview
Azo dyes are manufactured through a process called diazotization. A diazo component, or aromatic amine (AA), is treated with nitrous acid, which changes it into a diazonium component. The diazonium component reacts with a coupling component, such as phenol (carbolic acid) or another AA, to create the azo compound dye. Yellow azo dyes are the easiest colors to produce, while changes to the chemical structure result in various blacks, blues, reds, and violets. For example, the formula for CI Acid Red 74 is an aqueous solution of 4-aminonaphthalenesulfonic acid (naphthionic acid) added to a solution of 4-nitrobenzenediazonium chloride. The molecular formula for this single azo structure is C16H11N4NaO5S. Azo dyes can be adjusted by altering the chemical structure, which gives manufacturers a range of choices. If more electron-donating molecules are added to the azobenzene structure, this reduces the energy and moves the dye into a higher wavelength. (Wavelength, or light absorption, determines how the eye detects color.) This is known as the bathochromic effect or shift.
Some azo dyes owe their unique colors to a process called tautomerism. Tautomerism is the presence of two or more chemical compounds, existing in equilibrium, that differ only in the position of protons and electrons and are able to exchange protons. The hydrogen atom on the hydroxyl group of CI Acid Orange 7 (C16H11N2NaO4S), for example, migrates to the nitrogen atom, and vice versa. The color produced is usually the result of a longer wavelength (bathochromic shift).
Azo compounds may degrade, meaning the structure of the chemicals changes. This has caused some concern, because some resulting compounds are carcinogenic. Under some conditions, such as exposure to bacteria on the skin, the azo dyes in textiles may break down into AAs classified as Category 1 or 2 carcinogens (Category 1 are known or presumed human carcinogens; Category 2 are suspected human carcinogens). Researchers have been examining the possibility that these carcinogens could be absorbed through the skin, although hazards associated with these chemicals are most likely to affect workers involved in the manufacturing process. Most of the azo dyes listed as carcinogens are made using benzidine-based chemicals.
Concerns about possible dangers of azo dyes have led to legislation and calls for further study. Some azo dyes have been banned from clothing textiles in the European Union, and retailers have removed clothing containing azo dyes from the market. Concerns about the effects of colorings in foods have also led to further research. A University of Southampton study concluded that some children exhibited changes, such as concentration problems and impulsive behavior, after consuming additives and colorings. As a result of this study, the UK Food Standards Agency (FSA) recommended that six artificial colors be removed from food. This Southampton Six includes azo dyes Allura Red (E129), Camoisine (E122), Ponceau 4R (E124), Quinoline Yellow (E104), Sunset Yellow (E110), and Tartrazine (E102). In the United States, E104, E122, and E124, among other colors, are banned.
Bibliography
"Azo Dye." Museum of Fine Arts Boston, 29 Apr. 2016, cameo.mfa.org/wiki/Azo‗dye. Accessed 17 Oct. 2017.
Bavas, Josh. "Carcinogenic 'Azo' Dyes: Federal Government Considers Banning Chemicals Found in Some Clothing." ABC, 28 May 2014, www.abc.net.au/news/2014-05-28/govt-considers-banning-carcinogenic-dyes-more-found-in-clothing/5482040. Accessed 17 Oct. 2017.
Brüschweiler, Beat J., and Cédric Merlot. "Azo Dyes in Clothing Textiles Can Be Cleaved into a Series of Mutagenic Aromatic Amines Which Are Not Regulated Yet." Regulatory Toxicology and Pharmacology, vol. 88, Aug. 2017, pp. 214–26.
"Colorants." The Essential Chemical Industry, 18 Mar. 2013, www.essentialchemicalindustry.org/materials-and-applications/colorants.html. Accessed 17 Oct. 2017.
"Colors Family—Inks, Dyes and Pigments." World of Chemicals, www.worldofchemicals.com/407/chemistry-articles/colors-family-inks-dyes-and-pigments.html. Accessed 17 Oct. 2017.
"Food Additives and Behaviour in Children." University of Southampton, www.southampton.ac.uk/psychology/research/impact/food‗additives.page. Accessed 17 Oct. 2017.
"Food Color Additives Banned in the USA." Association of Food and Drug Officials, importedfoods.afdo.org/food-color-additives-banned-in-the-usa.html. Accessed 17 Oct. 2017.
Garfield, Simon. Mauve: How One Man Invented a Colour That Changed the World. Faber & Faber, 2000.
Yoquinto, Luke. "The Truth about Red Dye No. 2." Live Science, 7 Oct. 2011, www.livescience.com/35905-red-dye-no-2-truth.html. Accessed 17 Oct. 2017.